1. Field of the Invention
The present invention relates to integrated circuit fabrication and more particularly to methods for generating dense lines on a surface of a semiconductor wafer.
2. State of the Art
In recent years, dramatic progress has been made toward increasing the density of integrated circuits. In the area of gate arrays, for example, whereas only recently gate arrays offered about 10K to 20K usable gates, advanced gate arrays now offer upwards of 250K raw gates and 100K usable gates. In the area of semiconductor memories, 64 Mb DRAMs are presently available and 256 Mb DRAMs are on the horizon. Device line-widths have decreased from 1 um to 0.5 um and even smaller using specialized equipment. Typically, minimum resolution with a G-line stepper is about 0.6 um to 0.8 um. Such steppers cost approximately from 1 to 1.5 million dollars. Using an I-line stepper, minimum resolution is about 0.35 um to 0.45 um. The cost of an I-line stepper is about 2 to 2.5 million dollars. Eximer laser steppers offer a minimum resolution of about 0.25 um at a cost of about 3 million dollars.
Despite the foregoing advances, serious obstacles remain to achieving further increases in integration density. Unconventional lithographic methods such as E-beam and X-ray lithography face serious challenges. In conventional lithography using photoresists, an unavoidable tradeoff has been between keeping resist layers sufficiently thin to achieve good resolution while at the same time achieving step coverage, and leaving sufficient resist thickness to satisfy the requirements of etching processes that may exhibit only low to moderate selectivity. For example, factory specifications of a I-line stepper might specify a resolution of 0.38 um. The depth of focus of the machine, however, will typically be no more than 1 um. To achieve step coverage and satisfy the requirements of an etching process that does not exhibit extremely high selectivity, a resist layer may have to be 1.5 um thick, with the result that the specified resolution of 0.38 um cannot be achieved.
The invention allows this tradeoff between depth of focus on the one hand and step coverage/selectivity on the other hand to be avoided. The invention provides a method of generating equal lines and spaces of as small as 0.2 um using conventional G-line steppers having a small numerical aperture (approximately 0.43) at a cost of 1 million dollars or less.
Sharp Electronics has developed a phase-shifting wafer exposure technique using conventional masks. Using this technique, referred to as phase-shifting on the substrate (POST), extremely small lines on the order of 0.2 um or less may be achieved. The spaces using that technique are much bigger, however, than the lines. Two examples cited in Semiconductor lnternational, December 1991, page 16, refer to 0.15 um lines with a 0.5 um pitch and 0.2 um lines with 0.9 um pitch. According to the described technique, although line-widths can be made very small, density remains limited by the relatively large spaces between lines. The present method eliminates the problem of larger spaces, achieving higher density by generating equal-width lines and spaces.